1. Field of the Invention
This invention relates to the field of part holders for holding work pieces in a pattern generation and pattern printing and measuring devices.
2. Background Art
In the photo-lithographic fabrication of integrated circuits, films sensitive to radiant or particle energy are exposed in predetermined patterns to define circuit features. Often, the energy is passed through masks which contain the pattern, selectively exposing this photo resist film on a semiconductor body.
Various methods have been used in the past to generate such masks. For example, the mask patterns can be drawn at a large scale and subsequently photographically reduced to make the final masks. More recently, electron beams have been electrically directed to define patterns on a glass layer to define a mask. In addition to electron beams, lasers are now utilized to fabricate masks by directing the beam across a work piece in such a way as to define the pattern of interest. One such method is described in U.S. patent application No. 758,344 filed July 24, 1985, for "LASER PATTERN GENERATION APPARATUS" and assigned to the assignee of the present invention.
In order to accurately generate masks using a moving work piece and stationery beam, it is necessary to be able to determine the position of the piece relative to the beam in an xy plane. One method of determining the relative position of the work piece is differential interferometry and is decribed in the above referenced patent application. In differential interferometry, a signal is sent (in this case a laser beam) from a fixed source to mirrors mounted on the part holder and on the beam's lens. The signals are reflected back to a receiver near the fixed source and compared, enabling the calculation of relative distance between beam and part. In some applications, a differential interferometry assembly is mounted on each of the x and y axes to measure translational movement in those directions.
One problem with differential interferometry schemes is the inability to present a constant relationship between the work piece and a mirror used to indicate the position of the work piece. This limits the accuracy of the differential interferometry system. The part holder, which holds the work piece, is subject to thermal linear expansion. If a reference mirror is mounted on the part holder as well, the distance between the mirror and any point on the work piece will vary with temperature.
In the past, various solutions to this problem have been attempted. For example, the pattern generation apparatus has been maintained in a temperature controlled environment to eliminate linear thermal expansion. This method has the disadvantages of higher operating costs to maintain a constant temperature as well as incomplete environmental control which can result in significant residual errors.
Therefore, it is an object of the present invention to provide a means of maintaining a constant distance between a work piece and a reference point fixed in space with respect to the part holder and the reference mirrors used for measurement.
It is another object of the present invention to provide a part holder which will operate over a broad temperature range.
It is yet another object of the present invention to provide a means of eliminating translational variations resulting from differences in expansion characteristics at the part holder/substrate interface.
It is a further object of the present invention to eliminate measurement errors known as Abbe offset errors which result when measurements of features are made in a plane other than the plane which contains those features.
It is a still further object of the present invention to provide a part holder which is easily manufacturable.